The ozone with strong oxidability can effectively oxidize and decompose organic pollutants, inactivate pathogenic microorganisms, and particularly inactivate cryptosporidium and giardia (G&C for short). The G&C greatly harms the public health, and the efficiency for inactivating the G&C via chlorine disinfection is very low; the United States had one of the most serious infectious events in 1993, causing that 1.5 million people were infected, 403,000 people were in sick, and nearly one hundred people died; therefore, the United States enacted a strict G&C water quality standard, and the ozonation process was rapidly applied in the United States after 1990s of the last century.
One of the most serious problems of the ozonation technical application is to sterilize the byproducts, especially the bromate can directly harm the human health. When the bromine ions with certain concentration exist in water, the bromate can be generated during the ozone oxidation process. The bromate is recognized as 2B-grade potential carcinogen by the International Agency for Research on Cancer (IARC), the drinking water standards provided by the World Health Organization, the U.S. Environmental Protection Agency (EPA) and the European Union limit the bromate standard to be 10 μg/L.
There is a positive relationship between the bromate generation and the disinfection efficiency, the problem that how to guarantee the biosafety (inactivation of G&C) and the chemical safety (bromate control) of ozonation process effluent needs to be urgently solved today as the ozonation process is increasingly popularized.
The ozone disinfection efficiency depends on the ozone concentration C and the disinfection contact time T. The complex G&C detection with long period cannot be online monitored in practical application; the inactivation efficiency depends on the CT value (product of the ozone concentration C and the disinfection contact time T) according to the disinfection dynamics, thus the CT value becomes an important basis of the disinfection efficiency. The first method of improving the CT value is to increase the ozone concentration, which means that the ozone dosage is increased, the operation cost is increased accordingly, and meanwhile the bromate generation is increased; the second method is to improve the utilization efficiency of ozone in water, which is realized by optimizing the ozone contact tank.
The ozone contact tank is a treatment structure which makes the ozone gas diffuse to the treatment water, to fully contact with the water, and to implement reaction; a high-efficient ozone contact tank can improve the disinfection efficiency, reduce the bromate generation and save the operation cost.
There are generally two methods of dissolving the ozone into water at home and abroad, one is aeration method, and the other is pipeline mixing mode. The ozone needs to maintain a certain retention period in the contact tank after dissolving in water to realize full disinfection, the best theoretical method is that the water dissolved with ozone flows through the ozone contact tank via plug flow mode. In order to realize the plug flow mode, the ozone contact tank is always designed with a vertical partition plate; a typical vertical partition plate type ozone contact tank includes multiple compartments which are connected in series to each other; the water in the compartments generally has two flow directions, namely, rising and falling directions. Such type has simple design and construction, and is easy to maintain; however, lots of researches and engineering practices show that this contact tank has quite serious short stream and back-mixing phenomenon, and low hydraulic efficiency, which can greatly influence the disinfection effect; in order to guarantee the disinfection effect, the ozone dosage must be increased, the operation cost is increased, and meanwhile, the bromate generation is increased to harm the health of residents. Thereby, the key of solving the problems of the existing ozone contact tank is to improve the flow state in the ozone contact tank to be closer to the plug flow mode, and to improve the uniform distribution degree of the ozone concentration in the water.